Encode time.Time elements as Unix timestamps about go-edtf

[thisisaaronland]

Related:

> ./bin/parse 2000 '-1903/2020-01'
[{"start":{"edtf":"2000","lower":{"time":"2000-01-01T00:00:00Z","timestamp":{},"ymd":{"year":2000,"month":1,"day":1},"precision":64},"upper":{"time":"2000-01-01T23:59:59Z","ymd":{"year":2000,"month":1,"day":1},"precision":64}},"end":{"edtf":"2000","lower":{"time":"2000-12-31T00:00:00Z","ymd":{"year":2000,"month":12,"day":31},"precision":64},"upper":{"time":"2000-12-31T23:59:59Z","ymd":{"year":2000,"month":12,"day":31},"precision":64}},"edtf":"2000","level":0,"feature":"Date"}2021/01/08 12:26:14 Failed to encode EDTFDate, json: error calling MarshalJSON for type *time.Time: Time.MarshalJSON: year outside of range [0,9999]